In ecology, a disturbance is a change in environmental conditions that causes a pronounced change in an ecosystem. Disturbances often act quickly and with great effect, to alter the physical structure or arrangement of biotic and abiotic elements. A disturbance can also occur over a long period of time and can impact the biodiversity within an ecosystem. Ecological disturbances include fires, flooding, storms, insect outbreaks, trampling, human presence, earthquakes, plant diseases, infestations, volcanic eruptions, impact events, etc.

Not only invasive species can have a profound effect on an ecosystem, native species can also cause disturbance by their behavior. Disturbance forces can have profound immediate effects on ecosystems and can, accordingly, greatly alter the natural community’s population size or species richness. Because of these and the impacts on populations, disturbance determines the future shifts in dominance, various species successively becoming dominant as their life history characteristics, and associated life-forms, are exhibited over time.

The scale of disturbance ranges from events as small as a single tree falling, to as large as a mass extinction. Many natural ecosystems experience periodic disturbance that may broadly fall into a cyclical pattern. Ecosystems that form under these conditions are often maintained by regular disturbance. Wetland ecosystems, for example, can be maintained by the movement of water through them and by periodic fires. Different types of disturbance events occur in different habitats and climates with different weather conditions. Natural fire disturbances for example occur more often in areas with a higher incidence of lightning and flammable biomass, such as longleaf pine ecosystems in the southeastern United States. Wildfires, droughts, floods, disease outbreaks, changes in hydrology, tornadoes and other extreme weather, landslides, and windstorms are all examples of natural disturbance events that may form a cyclical or periodic pattern over time.

Other disturbances, such as those caused by humans, invasive species or impact events, can occur anywhere and are not necessarily cyclic. These disturbances can alter the trajectory of change within an ecosystem permanently. Extinction vortices may result in multiple disturbances or a greater frequency of a single disturbance.

Logging, dredging, conversion of land to ranching or agriculture, mowing, and mining are examples of anthropogenic disturbance. Human activities have introduced disturbances into ecosystems worldwide on a large scale, resulting in widespread range expansion and rapid evolution of disturbance-adapted species. Agricultural practices create novel ecosystems, known as agroecosystems, which are colonized by plant species adapted to disturbance and enforce evolutionary pressure upon those species. Species adapted to anthropogenic disturbance are often known as weeds.

Another example of anthropogenic disturbance is controlled burns used by Native Americans to maintain fire-dependent ecosystems. These disturbances helped maintain stability and biodiversity in ecosystems, enhancing overall ecosystem health and functioning.

Anthropogenic climate change is considered a major source of change in future successional trajectories of ecosystems.

Immediately after a disturbance there is a pulse of recruitment or regrowth under conditions of little competition for space or other resources. After the initial pulse, recruitment slows since once an individual plant is established it is very difficult to displace. Because scale-dependent relationships are ubiquitous in ecology, the spatial scale modulates the effect of disturbance on natural communities. For example, seed dispersal and herbivory may decrease with distance from the edge of a burn. Consequently, plant communities in the interior areas of large fires respond differently than those in smaller fires. Although disturbance types have varied on ecosystems, spatial scale likely influences ecological interactions and community recovery from all cases because organisms differ in dispersal and movement capabilities.